Heat pipe diode assembly and method

ABSTRACT

A heat pipe diode assembly is provided that includes at least two heat pipes and at least two working fluid reservoirs, with one working fluid reservoir arranged in fluid communication with each the heat pipes. A cold finger is arranged in thermal engagement with each of the heat pipes. A device to be cooled is arranged in thermal engagement with a portion of each of the heat pipes. Another embodiment provides a heat pipe diode assembly formed in accordance with the invention includes three heat pipes and six working fluid reservoirs, with two working fluid reservoirs arranged in fluid communication with each of the three heat pipes. Three cold fingers are provided, with one arranged in thermal engagement with each of the three heat pipes. A device to be cooled is arranged in thermal engagement with a portion of each of the three heat pipes. Methods are also provided for operation of a heat pipe diode assembly.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from co-pending Provisional Patent Application Serial No. 60/365,322, filed Mar. 18, 2002, and entitled CRISM DIODE HEAT PIPE ASSEMBLY.

FIELD OF THE INVENTION

[0002] The present invention generally relates to cooling systems for radiation detectors, and more particularly to heat pipe-based cooling systems for radiation detectors.

BACKGROUND OF THE INVENTION

[0003] A radiation detector is a device that produces an output signal which is a function of an amount of radiation that is incident upon an active region of the radiation detector. Infrared detectors are radiation detectors that are sensitive to radiation in the infrared region of the electromagnetic spectrum. An infrared detector may be, for example, a thermal detector. A thermal detector detects radiation based upon a change in the temperature of an active region of the detector due to absorption of radiation incident upon the detector. The active region of a thermal detector is very often maintained at a very low temperature (typically a cryogenic temperature) relative to the surrounding environment. Thermal imaging sensors may include a plurality of thermal detectors arranged in an array that detect a representation of an object by the objects' thermal emissions. In particular, energy emitted by an object may depend upon numerous quantities such as, for example, the emissitivity and the temperature of the object. Infrared thermal sensors typically detect one or both of these quantities and use the detected information to produce an object image that may be viewed, for example, on a display.

[0004] U.S. Pat. No. 3,851,173 describes one example of an infrared detector comprising an envelope, a cold finger in the envelope and at least one detector element mounted on an end of the cold finger so as to be cooled by the cold finger in operation of the detector. The envelope has an outer wall extending around the cold finger and an infrared-transmissive window facing the detector-element end of the cold finger.

[0005] U.S. Pat. No. 5,235,817, discloses a radiation detector cryogenic cooling apparatus having a plurality of nested thermally conductive elongate tubes that reduce thermal radiation as a parasitic input to the detector and cold finger, allowing higher efficiency and lower detector temperatures to be achieved.

[0006] In a number of space surveillance systems, detectors are mounted on a focal plane. These detectors are often of the type which require cooling. The prior art includes the mounting of numerous detectors in an array on a focal plane; and in order to cool the array, heat pipes are mounted to the rear of the focal plane array. Certain types of these detectors require operation at extremely low temperatures. At these temperatures, active cryocoolers are often employed because of the limited heat rejection per unit area available with passive cryoradiators and the subsequent large required radiator areas. However, many active cryocoolers for space surveillance systems have less projected operating life than desired. Providing two or more cryocoolers, with only one operating at a given time, would theoretically double the operating life of a surveillance system. The non-operating cryocooler must be thermally decoupled from a cryogenic loop to prevent excessive heat leakage. Thermal switches based on heat pipe technology have been proposed to permit switching between coolers while preventing heat transfer from a warmer inactive cooler. However, known switches offer inadequate performance.

[0007] In U.S. Pat. No. 5,111,874, issued to Kosson, a heat pipe-based thermal switch is provided that includes a single vapor channel communicating with two parallel formed liquid channels. Each of the liquid channels is connected to a respective liquid channel in a dual heat sink assembly which thermally contacts a cryocooler. If a first cryocooler becomes operative, liquid in the heat pipe thermal switch will fill a liquid channel communicating with the heat sink liquid channel which thermally contacts an operative cryocooler. The other heat pipe thermal switch liquid channel is filled with vapor so as to thermally decouple its respectively connected thermal cooler from the system.

[0008] It would be advantageous to provide a heat pipe-based cooling system that exhibits diode-like thermal properties when used in connection with focal plane array based radiation detectors and the like.

SUMMARY OF THE INVENTION

[0009] The present invention provides a heat pipe diode assembly including at least two heat pipes and at least two working fluid reservoirs, with one working fluid reservoir arranged in fluid communication with each of the heat pipes. A cold finger is arranged in thermal engagement with a portion of each of the heat pipes, and a device to be cooled is arranged in thermal engagement with another portion of each of the heat pipes. In this way, when the cold finger associated with a first heat pipe is actively cooled by, e.g., an associated cryocooler, and the other cold fingers are left inactive at or near ambient temperature, so that the working fluid reservoir and condenser end of the active heat pipe will cool to the operating temperature. Working fluid will condense and initiate heat pipe action which, in turn, cools the device to be cooled, e.g., a focal plane array of an infrared detector. The working fluid reservoirs that are attached to the inactive cold fingers remain relatively warm. However, the evaporator ends of the inactive heat pipes will cool because they are thermally engaged with the same device to be cooled as the active heat pipe. When the volume of the working fluid reservoirs and initial fluid pressures are selected within predetermined ranges, the pressure will remain supercritical and only cold, supercritical “fluid” will collect in the evaporator ends of the inactive heat pipes. Because this supercritical fluid can only exist as a single phase, the inactive heat pipes remain inactive.

[0010] A method for forming a redundant heat pipe diode assembly is also provided by which a first working fluid reservoir is connected in fluid communication with a condenser end of a first heat pipe. The condenser end of the first heat pipe is cooled while an evaporator end is arranged in thermal engagement with a device to be cooled. The first fluid reservoir is cooled to a temperature that is substantially the same as the temperature of the condenser end of the first heat pipe. A second working fluid reservoir is connected in fluid communication with a condenser end of a second heat pipe. The condenser end of the second heat pipe and the second working fluid reservoir are maintained at an ambient temperature that is above the temperature of the condenser end of the first heat pipe while an evaporator end of the second heat pipe is arranged in thermal engagement with the device to be cooled.

[0011] In an alternative embodiment, a heat pipe diode assembly formed in accordance with the invention includes three heat pipes and six working fluid reservoirs, with two working fluid reservoirs arranged in fluid communication with each of the three heat pipes. Three cold fingers are provided, with one arranged in thermal engagement with each of the three heat pipes. A device to be cooled is arranged in thermal engagement with a portion of each of the three heat pipes. In this way, the need for supercritical pressures is obviated. Each heat pipe has two working fluid reservoirs of half the size that would be required to contain the internal pressure while at ambient conditions. The working fluid reservoirs of each heat pipe are in thermal engagement with a respective cold finger. Here again, when the cold finger associated with the heat pipe that is actively being cooled by, e.g., an associated cryo-cooler, the other two cold fingers are inactive and are at or near ambient temperature. Working fluid from the other two heat pipes will condense in their respective reservoirs that are thermally engaged with the cooled cold finger. The condensation in these working fluid reservoirs will deprive the other two, inactive heat pipes of any fluid, thereby placing them in an “off” condition. However, both reservoirs of the active heat pipe will be warm (above their critical temperature) because they are attached to inactive cold fingers. Thus, fluid can condense in the active heat pipe, initiate heat pipe action, and cool the device connected to the evaporation ends of the heat pipes. There is no need to maintain the pressure above a “critical” pressure when in an “off” condition, thus the more desirable oxygen can be used as the working fluid.

[0012] An alternative method for forming a redundant heat pipe diode assembly is also provided by which a first working fluid reservoir is connected in fluid communication with a condenser end of a first heat pipe and a second working fluid reservoir is connected in fluid communication with the condenser end of the first heat pipe. A third working fluid reservoir is connected in fluid communication with a condenser end of a second heat pipe and a fourth working fluid reservoir is connected in fluid communication with the condenser end of the second heat pipe. A fifth working fluid reservoir is connected in fluid communication with a condenser end of a third heat pipe and a sixth working fluid reservoir is connected in fluid communication with the condenser end of the third heat pipe. The condenser end of the first heat pipe is cooled while the evaporator ends of all of the heat pipes are arranged in thermal engagement with a device to be cooled. The sixth working fluid reservoir is cooled to a temperature that is substantially the same as the temperature of the condenser end of the first heat pipe. The fourth working fluid reservoir is cooled to a temperature that is substantially the same as the temperature of the condenser end of the first heat pipe. The condenser ends of the second and third heat pipes are maintained at an ambient temperature above the temperature of the condenser end of the first heat pipe. Each of the first, the second, the third, and the fifth working fluid reservoirs are maintained at an ambient temperature above the temperature of the condenser end of the first heat pipe. Thus, fluid can condense in the active heat pipe, initiate heat pipe action, and cool the device connected to the evaporation ends of all of the heat pipes while the remaining inactive heat pipes are maintained in an “off” condition, essentially starved of working fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] These and other features and advantages of the present invention will be more fully disclosed in, or rendered obvious by, the following detailed description of the preferred embodiments of the invention, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

[0014]FIG. 1 is a schematic side elevational view of a heat pipe diode assembly formed in accordance with the present invention; and

[0015]FIG. 2 is a schematic side elevational view of an alternative embodiment of a heat pipe diode assembly formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] This description of preferred embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description of this invention. The drawing figures are not necessarily to scale and certain features of the invention may be shown exaggerated in scale or in somewhat schematic form in the interest of clarity and conciseness. Reference numerals with subscripts refer to a particular structural embodiment, while those same reference numerals without subscripts refer to a general structure that includes the particular embodiment. In the description, relative terms such as “horizontal,”“vertical,”“up,”“down,”“top” and “bottom” as well as derivatives thereof (e.g., “horizontally,”“downwardly,”“upwardly,” etc.) should be construed to refer to the orientation as then described or as shown in the drawing figure under discussion. These relative terms are for convenience of description and normally are not intended to require a particular orientation. Terms including “inwardly” versus “outwardly,”“longitudinal” versus “lateral” and the like are to be interpreted relative to one another or relative to an axis of elongation, or an axis or center of rotation, as appropriate. Terms concerning attachments, coupling and the like, such as “connected” and “interconnected,” refer to a relationship wherein structures are secured or attached to one another either directly or indirectly through intervening structures, as well as both movable or rigid attachments or relationships, unless expressly described otherwise. The term “operatively connected” is such an attachment, coupling or connection that allows the pertinent structures to operate as intended by virtue of that relationship. In the claims, means-plus-function clauses are intended to cover the structures described, suggested, or rendered obvious by the written description or drawings for performing the recited function, including not only structural equivalents but also equivalent structures.

[0017] A heat pipe diode assembly 5 formed in accordance with the present invention generally includes at least two heat pipes 8, at least two working fluid reservoirs 10, at least two cold finger assemblies 12, and a device to be cooled, e.g., a focal plane array (FPA) 14. Very often three heat pipes 8 a,8 b,8 c, three working fluid reservoirs 10 a, 10 b, 10 c, and three cold finger assemblies 12 a, 12 b, 12 c are employed. A preferred application for heat pipe diode assembly 5 is to provide flexible, thermal links between an infra-red imaging focal plane and three RICOR cryo-coolers (not shown). During operation of an infra-red imaging system, only one of the three cryo-coolers will be active at any time; heat pipe 8 a connecting active cooler 12 a to focal plane array 14 will be in an “on” condition, while the remaining two heat pipes will be in an “off” condition, effectively insulating focal plane array 14 from the “warm” coolers 12 b, 12 c. Although the present invention does not exclude “active” commanding of the heat pipes from the “on” to the “off” condition and visa versa, “passive” switching in response to the operation of the coolers has been found to be more desirable.

[0018] In one preferred embodiment of the invention, heat pipe diode assembly 5 comprises three heat pipes 8 a,8 b,8 c(FIG. 1). More particularly, each heat pipe 8 comprises an elongate, hermetically sealed tube 17 having an evaporator end 19, a condenser end 21, and a central passageway or vapor chamber 24. Evaporator end 19 is thermally engaged with a portion of FPA 14, and condenser end 21 is thermally engaged with a portion of cold finger assembly 12. Heat pipes 8 a,8 b,8 c often have equal lengths and identical working fluid reservoirs 10. Evaporator ends 19 often comprise a flat interface that matches the specified mounting area on FPA 14. A flexible link, either a strap or a pad, provides the required flexibility and CTE matching. Working fluid reservoirs 10 of each heat pipe 8 are in thermal contact with a cold finger assembly 12. A wick 27 is disposed within vapor chamber 24 along with a suitable working fluid 29. Heat pipe 8 a,8 b,8 c are most often formed from thermally conductive material, e.g., copper, aluminum, steel, or their alloys. Oxygen or nitrogen are both preferred working fluids 29 for cryogenic applications of heat pipe diode assembly 5. Wick 27 is often formed from a single or double layer of mesh or screen made of a suitable material, e.g., stainless steel, or the like, and is arranged so as to line the interior surface of tube 17.

[0019] The design of each heat pipe 8 is often driven by the heat transport requirements of FPA 14. These are very low, even for the longest pipe the required “QL” product is less than 0.12 W-m at 80° K. This requirement is achieved by appropriate selection of the wick design. In the present invention, the choice of the heat pipe envelope is driven by the need for a high resistance in a non-functioning, “off” condition. In one embodiment, heat pipe 8 is formed from a thin walled stainless steel pipe with a multi-layer screen wick 27, where the heat pipe envelope is 6.35 mm outer diameter ×0.25 mm wall thickness(0.250″×0.010″). Wick 27 may comprise two layers of fifty mesh stainless screen. In a preferred embodiment, each heat pipe 8 may have an overall length of from 9.3 cm to 21 cm. The evaporator footprint is often about 2.4 cm². Each heat pipe 8 comprises a heat transport capability of about 500 mW at 80° K, with an “on” thermal conductance of about 0.3 W/K and an “off” thermal resistance of about 1500 K/W.

[0020] Working fluid reservoirs 10 a,10 b,10 c each comprise a hollow vessel that is thermally engaged with a portion of a corresponding cold finger assembly 12 a, 12 b, 12 c, and in fluid communication with a condenser end 21 of a heat pipe 8 a,8 b,8 c. Working fluid reservoirs 10 a,10 b,10 c will often have a volume of less than 100 cc. A tube 32, extending from condenser end 21 to reservoir 10, provides a conduit through which working fluid 27 communicates between the interior of reservoir 10 and vapor chamber 24. Tube 32 is often lined with a wicking material, e.g., wick 27. Working fluid reservoirs 10 a,10 b,10 c help to maintain the internal pressure within each heat pipe 8 at acceptable limits when the system is near ambient temperature.

[0021] Cold finger assembly 12 includes a cold finger 35 that is thermally engaged with a conventional cooler, e.g., a RICOR cryocooler (not shown). As shown in FIGS. 1 and 2, one preferred heat pipe diode assembly 5 comprises three cold finger assemblies 12 a,12 b,12 c having three cold fingers 35 a,35 b,35 c. FPA 14 is thermally conductively attached to each evaporator end 19, and a cold finger 35 is thermally conductively attached to each condenser end 21 of each heat pipe 8. In order to reduce parasitic heat input to the system, cold finger assembly 12 is generally insulated from the environment in a conventional Dewar, having a vacuum space within a sealed envelope or chamber (not shown). A cooling element is provided in the chamber and serves to cool cold finger 35 during operation of the system. In order to achieve the desired cryogenic temperatures, around 80° K-87° K at FPA 14 and cold finger 35, the Dewar is typically filled with liquid nitrogen, which exists at 77° K in the liquid state at standard pressure, and which is allowed to vaporize, so as to provide cooling of cold finger 35 in cold-finger assembly 12.

[0022] The present invention utilizes the physical principle that a fluid cannot exist in a two-phase state, at any temperature, as long as its pressure exceeds a critical pressure. Thus in this state, a heat pipe cannot function —it remains in an “off” condition —as long as the pressure within its vapor chamber is higher than a critical pressure. In the present invention, condenser end 21 of any inactive heat pipe 8 is blocked with a single phase, supercritical fluid (either oxygen or nitrogen) so that circulation within heat pipe 8 is inhibited anywhere in the pipe. The extent of this single phase, supercritical fluid is schematically represented in FIG. 1, by a line adjacent to evaporator ends 19 in heat pipes 8 b and 8 c. This effect increases the thermal resistance in the “off” mode, thus achieving “diode action.”

[0023] Referring to FIG. 1, when cold finger 35 a is actively being cooled by its associated cryo-cooler, the other two cold fingers 35 b,35 c are inactive and are at or near ambient temperature. Working fluid reservoir 10 a and condenser end 21 of active heat pipe 8 a will cool to the operating temperature; working fluid will condense and initiate heat pipe action which, in turn, cools FPA 14. Working fluid reservoirs 10 b,10 c that are attached to the inactive cold fingers 35 b,35 c remain relatively warm. However, evaporator ends 19 of the two inactive heat pipes 8 b,8 c will cool because they are attached in thermal engagement to the same FPA 14 as active heat pipe 8 a. When the volume of working fluid reservoir 10 a and initial fluid pressures are selected within predetermined ranges, the pressure will remain supercritical and only cold, supercritical “fluid” will collect in evaporator ends 19 of inactive heat pipes 8 b,8 c. Because this supercritical fluid can only exist as a single phase, the heat pipes 8 b and 8 c remain inactive.

[0024] The present invention will work in principle with any cryogenic working fluid. However, in order to maintain the pressure at reasonable values, nitrogen is a preferred choice, since it comprises a critical pressure of 3.39 MPa (490 psi) while that of oxygen is 5.04 MPa (732 psi). Except for the higher critical pressure, oxygen would be the preferred fluid because of its better transport properties which permit a simpler wick structure or, conversely, have higher performance margins.

[0025] Referring to FIG. 2, an alternative embodiment of the present invention utilizes two working fluid reservoirs per heat pipe to achieve the desired “on” condition/ “off” condition diode effect. One embodiment of heat pipe diode assembly 50 comprises at least three heat pipes 52 a,52 b,52 c. Each heat pipe 52 comprises an elongate, hermetically sealed tube 17 having an evaporator end 19, a condenser end 21, and a central passageway or vapor chamber 24. Evaporator end 19 is thermally engaged with a portion of FPA 14, and condenser end 21 is thermally engaged with a portion of cold finger assembly 12. A wick 27 is disposed within vapor chamber 24 along with a suitable working fluid 29. Heat pipes 52 a,52 b,52 c are most often formed from thermally conductive material, e.g., copper, aluminum, steel, or their alloys. Oxygen or nitrogen are preferred working fluids 27 for cryogenic applications of heat pipe diode assembly 50. Wick 27 is often formed from a single or double layer of mesh or screen made of a suitable material, e.g., stainless steel or the like, and is arranged so as to line the interior surface of tube 17.

[0026] The design of each heat pipe 52 a,52 b,52 c is often driven by the heat transport requirements of FPA 14. These are very low, even for the longest pipe the required “QL” product is less than 0.12 W-m at 80° K. This requirement is achieved by appropriate selection and design of wick 27. In this embodiment of the invention the choice of the heat pipe envelope is once again driven by the need for a high thermal resistance in a non-functioning, “off” condition. Thus, heat pipes 52 a,52 b,52 c are each formed from a thin walled stainless steel pipe with a multi-layer screen wick 27, where the heat pipe envelope is about 6.35 mm outer diameter×0.25 mm wall thickness(0.250″×0.010″). Wick 27 may comprise two layers of fifty mesh stainless screen. In one preferred embodiment, each heat pipe 52 a,52 b,52 c may have an overall length of from about 9.3 cm to 21 cm. The evaporator footprint is often about 2.4 cm². Each heat pipe 52 a,52 b,52 c comprises a heat transport capability of about 500 mW at 80° K, with an “on” thermal conductance of about 0.3 W/K and an “off” thermal resistance of about 1500 K/W.

[0027] Unlike heat pipe diode assembly 5, heat pipe diode assembly 50 includes two working fluid reservoirs 60 for each heat pipe 52. Each working fluid reservoir 60 comprises a hollow vessel that is thermally engaged with a portion of cold finger assembly 12, and in fluid communication with a condenser end 21 of a heat pipe 52, according to a predetermined fluid communication scheme. Working fluid reservoirs 60 will also often have a volume of less than 100 cc. Pairs of tubes 62 extend from each condenser end 21 to selected pairs of working fluid reservoirs 60 according to the predetermined fluid communication scheme so as to provide conduits through which working fluid 27 communicates between the interior of working fluid reservoirs 60 and vapor chambers 24. Each tube 62 comprises un-wicked central bore of small diameter (typically 1.5 mm ({fraction (1/16)}″) outer diameter). Working fluid reservoirs 60 also help to maintain the internal pressure with each heat pipe 52 at acceptable limits when heat pipe diode assembly 50 is near ambient temperature.

[0028] Here again, cold finger assemblies 12 a,12 b,12 c include cold fingers 35 a,35 b,35 c which are also thermally engaged with a cooler as in heat pipe diode assembly 5. FPA 14 is thermally conductively attached to evaporator end 19, and each cold finger 35 is thermally conductively attached to condenser end 21, of each heat pipe 52.

[0029] In one preferred embodiment, the predetermined fluid communication scheme is arranged as follows. A pair of working fluid reservoirs 60 (identified by reference labels R_(c)-1 and R_(b)-1 in FIG. 2) are mounted on cold finger 35 a, and are also thermally engaging condenser end 21 of heat pipe 52 a. Working fluid reservoirs R_(c)-1 and R_(b)-1 are also arranged in fluid communication, via a first pair of tubes 62, with heat pipes 52 c and 52 b. Another pair or fluid reservoirs 60 are mounted on cold finger 35 b that is thermally engaging condenser end 21 of heat pipe 52 b and are also in fluid communication, via a second pair of tubes 62, with heat pipes 52 c and 52 a and are thus designated R_(c)-2 and R_(a)-1. A further pair of working fluid reservoirs 60 are mounted on cold finger 35 c that is thermally engaging condenser end 21 of heat pipe 52 c. This pair of working fluid reservoirs 60 are in fluid communication, via a third pair of tubes 62, with heat pipes 52 b and 52 a, and are thus designated R_(b)-2 and R_(a)-2.

[0030] Heat pipe diode assembly 50 eliminates the need for supercritical pressures and does not require a wicked connection between condenser ends 21 and working fluid reservoirs 60. Each heat pipe 52 a,52 b,52 c has two working fluid reservoirs 60 of half the size that would be required to contain the internal pressure while at ambient conditions. Working fluid reservoirs 60 of each heat pipe 52 a,52 b,52 c are in thermal engagement with a respective cold finger 35. Here again, when cold finger 35 a associated with, e.g., heat pipe 52 a is actively being cooled by its associated cryo-cooler, the other two cold fingers 35 b, 35 c are inactive and are at or near ambient temperature. Working fluid from the other two heat pipes 52 b,52 c will condense in the two reservoirs R_(c)-1 and R_(b)-1 that are in thermal engagement with cold finger 35 a. The condensation in working fluid reservoirs R_(c)-1 will deprive the other two heat pipes 52 b,52 c of any fluid, thereby placing them in an “off” condition. However, both reservoirs of heat pipe 52 a a will be warm (above their critical temperature) because they are in thermal engagement with the inactive cold fingers 35 b, 35 c. Thus, fluid can condense in active heat pipe 52 a, initiate heat pipe action and cool FPA 14. Because there is no fluid transported through connecting tubes 62, but only vapor, tubes 62 need not be lined with a wick, and can have small diameters. Also, there is no need to maintain the pressure above a “critical” pressure when in an “off” condition, thus the more desirable oxygen can be used as the working fluid. It should be noted that if only two instead of three coolers were involved, each heat pipe 52 a,52 b,52 c would require only a single working fluid reservoir 60 that would then be attached to anther cooler.

[0031] It is to be understood that the present invention is by no means limited only to the particular constructions herein disclosed and shown in the drawings, but also comprises any modifications or equivalents within the scope of the claims. 

What is claimed is:
 1. A heat pipe diode assembly comprising: at least two heat pipes; at least two working fluid reservoirs, one arranged in fluid communication with each of said heat pipes; at least two cold fingers, one arranged in thermal engagement with each of said heat pipes; and a device to be cooled arranged in thermal engagement with a portion of each of said heat pipes.
 2. A heat pipe diode assembly according to claim 1 wherein a tube having a wicked interior passageway interconnects one of said at least two heat pipes to one of said at least two working fluid reservoirs.
 3. A heat pipe diode assembly according to claim 2 comprising three heat pipes.
 4. A heat pipe diode assembly according to claim 1 wherein said device to be cooled comprises a focal plane array.
 5. A heat pipe diode assembly according to claim 1 wherein each of said heat pipes includes an evaporator end and a condenser end so that a cold finger thermally engages said condenser end and said device thermally engages said evaporator end.
 6. A heat pipe diode assembly according to claim 5 wherein said working fluid essentially comprises at least one of oxygen and nitrogen.
 7. A heat pipe diode assembly comprising: three heat pipes; three working fluid reservoirs, one arranged in fluid communication with each of said three heat pipes; three cold fingers, one arranged in thermal engagement with each of said three heat pipes; and a device to be cooled arranged in thermal engagement with a portion of each of said three heat pipes.
 8. A heat pipe diode assembly according to claim 7 wherein each of said three heat pipes is connected in fluid communication with a corresponding one of said three working fluid reservoirs by at least one tube.
 9. A heat pipe diode assembly according to claim 7 wherein said device to be cooled comprises a focal plane array.
 10. A heat pipe diode assembly according to claim 7 wherein each of said heat pipes includes an evaporator end and a condenser end so that one of said three cold fingers thermally engages said condenser end and said device thermally engages said evaporator end of each of said heat pipes.
 11. A heat pipe diode assembly according to claim 10 wherein said working fluid essentially comprises at least one of oxygen and nitrogen.
 12. A heat pipe diode assembly comprising: three heat pipes; six working fluid reservoirs, two arranged in fluid communication with each of said three heat pipes; three cold fingers, one each arranged in thermal engagement with two of said six working fluid reservoirs and one of said heat pipes; and a device to be cooled arranged in thermal engagement with a portion of each of said three heat pipes.
 13. A heat pipe diode assembly according to claim 12 wherein each heat pipe is connected in fluid communication with two of said six working fluid reservoirs by a tube.
 14. A heat pipe diode assembly according to claim 13 wherein no more than two of said six working fluid reservoirs is cooled.
 15. A heat pipe diode assembly comprising: three heat pipes each including a condenser end and an evaporator end; a first working fluid reservoir arranged in fluid communication with a first heat pipe; a second working fluid reservoir arranged in fluid communication with a second heat pipe; a third working fluid reservoir arranged in fluid communication with a third heat pipe; three cold fingers, one arranged in thermal engagement with said condenser end of each of said heat pipes; and a device to be cooled arranged in thermal engagement with said evaporator end of each of said heat pipes.
 16. A heat pipe diode assembly according to claim 15 wherein said working fluid reservoirs are connected to said heat pipes by a tube having a wicked interior surface.
 17. A heat pipe diode assembly according to claim 15 wherein said device to be cooled comprises a focal plane array.
 18. A heat pipe diode assembly according to claim 15 wherein said heat pipes comprise a working fluid essentially comprising at least one of oxygen and nitrogen.
 19. A method for forming a redundant heat pipe diode assembly comprising: connecting a first working fluid reservoir in fluid communication with a condenser end of a first heat pipe; cooling said condenser end of said first heat pipe while an evaporator end is arranged in thermal engagement with a device to be cooled; cooling said first fluid reservoir to a temperature that is substantially the same as the temperature of said condenser end of said first heat pipe; connecting a second working fluid reservoir in fluid communication with a condenser end of a second heat pipe; maintaining said condenser end of a second heat pipe and said second working fluid reservoir at an ambient temperature above said temperature of said condenser end of said first heat pipe while an evaporator end of said second heat pipe is arranged in thermal engagement with said device to be cooled.
 20. A method for forming a redundant heat pipe diode assembly comprising: connecting a first working fluid reservoir in fluid communication with a condenser end of a first heat pipe; connecting a second working fluid reservoir in fluid communication with said condenser end of said first heat pipe; connecting a third working fluid reservoir in fluid communication with a condenser end of a second heat pipe; connecting a fourth working fluid reservoir in fluid communication with said condenser end of said second heat pipe; connecting a fifth working fluid reservoir in fluid communication with a condenser end of a third heat pipe; connecting a sixth working fluid reservoir in fluid communication with said condenser end of said third heat pipe; cooling said condenser end of said first heat pipe while an evaporator end is arranged in thermal engagement with a device to be cooled; cooling said sixth working fluid reservoir to a temperature that is substantially the same as the temperature of said condenser end of said first heat pipe; cooling said fourth working fluid reservoir to a temperature that is substantially the same as the temperature of said condenser end of said first heat pipe; maintaining said condenser ends of said second and third heat pipes and at an ambient temperature above said temperature of said condenser end of said first heat pipe while an evaporator end of said second and third heat pipes is arranged in thermal engagement with said device to be cooled; and maintaining each of said first, said second, said third, and said fifth working fluid reservoirs at an ambient temperature above said temperature of said condenser end of said first heat pipe.
 21. A heat pipe diode assembly comprising: two heat pipes; two working fluid reservoirs, one arranged in fluid communication with each of said heat pipes; two cold fingers, one arranged in thermal engagement with each of said heat pipes; and a device to be cooled arranged in thermal engagement with a portion of each of said heat pipes so that a working fluid pressure within one of said two heat pipes remains supercritical.
 22. A heat pipe diode assembly according to claim 21 wherein each of said heat pipes comprise an evaporator portion in thermal engagement with said device to be cooled so that said supercritical working fluid collects in said evaporator end of an inactive one of said two heat pipes.
 23. A heat pipe diode assembly according to claim 21 wherein a tube having a wicked interior passageway interconnects one of said at least two heat pipes to one of said at least two working fluid reservoirs.
 24. A heat pipe diode assembly according to claim 21 wherein said device to be cooled comprises a focal plane array.
 25. A heat pipe diode assembly according to claim 21 wherein each of said heat pipes includes an evaporator end and a condenser end so that a cold finger thermally engages said condenser end and said device thermally engages said evaporator end so that said supercritical working fluid collects in said evaporator end of an inactive one of said two heat pipes.
 26. A heat pipe diode assembly according to claim 25 wherein said working fluid essentially comprises at least one of oxygen and nitrogen.
 27. A heat pipe diode assembly comprising: at least two heat pipes; at least two working fluid reservoirs, one arranged in fluid communication with each of said heat pipes; at least two cold fingers, one arranged in thermal engagement with each of said heat pipes; and a device to be cooled arranged in thermal engagement with a portion of each of said heat pipes so that a working fluid pressure within all but one of said at least two heat pipes remains supercritical. 